The use of air for condensation of turbine steam is a proven and frequently used practice. In the case of direct air condensation, the turbine steam is condensed in ribbed pipe elements connected in parallel and the condensate is returned to the feed water circuit. The ribbed pipe elements are under vacuum internally, wherein the noncondensable gases are sucked out. The flow of cooling air is generally generated by means of ventilators and, more rarely, by means of natural ventilation.
The surface condensers are typically constructed in a roof-type construction (A-arrangement), as it is called, with diagonally arranged cooling pipes. In this case, the cooling pipes form the sides of a triangle at whose base are arranged ventilators. As a rule, the cooling pipes are combined in groups or rows. In so doing, cooling pipes in condenser operation and cooling pipes in dephlegmator operation are often coupled.
In cooling pipes in condenser operation (condenser pipes), the condensate flows in the direction of the steam guided through the cooling pipes (parallel flow condenser), whereas in cooling pipes in dephlegmator operation (dephlegmator pipes), the condensate flows in the opposite direction to the steam (countercurrent condenser). The cooling pipes in dephlegmator operation serve in particular to counter the risk of freezing.
The combination of parallel flow condensers and countercurrent condensers is known in the art, for example, from DE-PS 1 188 629.
In this case, dephlegmator pipes are connected downstream of the condenser pipes. At the same time, they are divided by groups into cooling sectors in such a way that at least a portion of the groups connected in condenser operation can be switched off on the air side in the winter months when operating under partial load and at outside temperatures below freezing in order to precipitate the steam predominantly in the groups connected in dephlegmator operation. Although countercurrent condensers have a poorer efficiency than the parallel flow condensers, they have the advantage that they do not freeze even under partial loading due to the continual contact between the downward running condensate and the upward flowing steam.
The so-called condensation end of the steam is accordingly located in the countercurrent condenser, so that an undercooling of the condensate is prevented in general. Regulation is carried out in this case by switching off individual cooling sectors or by changing the flow of cooling air.
DE 28 45 181 A1 discloses a surface condenser in which a portion of the cooling pipes has an inner dividing wall. In this way, two channels are formed in the cooling pipe, one of which serves for the conduction of steam and the flowing off of the condensate, while the other serves to suck out air and other noncondensable components.
It is further known in the art from the brochure by the Hamon company, "Vacuum Steam Air Condenser--The H. S. Integrated System", to arrange condenser pipes and dephlegmator pipes in a pipe bundle. In this case, the condenser pipes are arranged in the first rows of pipes and the dephlegmator pipes are arranged in a row of pipes which is connected downstream on the air side.
In the dephlegmator pipes, the steam flows in from below and is precipitated in the counterflow to form condensate which runs off downward. As was already mentioned, this has the advantage for operation that the condensate is always maintained approximately at the equilibrium temperature by the steam and an undercooling and the risk of freezing are therefore prevented.
However, in practical operation this dividing up has the disadvantage that the steam velocity is very high particularly in the case of long dephlegmator pipes which have economical advantages because of the large amount of condensable steam in every dephlegmator pipe. This hinders the running off of condensate. This can lead to a condensate back-up or swallowing in the dephlegmator pipes. This swallowing occurs when the steam velocities on entering the dephlegmator pipes are so high that they carry out a screen-like or buffer-like retention of the condensate flowing off in the counterflow or, in some cases, push the condensate upward. This leads to the formation of a water plug which runs downward in a gushing manner when a maximum load is exceeded. This impairs the condensation performance of a surface condenser. In particular, the swallowing results in large pressure losses and in pressure fluctuations in the surface condenser with detrimental effects on operation.
It has been demonstrated that the most difficult operating conditions exist at the pipe entrance to the dephlegmator pipes, where steam and condensate flow against one another. The pipe entrance is also the narrowest point with the highest steam velocity and the highest condensate velocity. Moreover, the gas flow at the pipe entrance is disrupted because of the transition into the dephlegmator pipes with the consequent flow bottleneck. This increases the influence of friction on the condensate running off and increases the tendency toward back-up. Farther up in the dephlegmator pipe, the steam velocity becomes more homogeneous and decreases. Consequently, the friction resistance also decreases.